In endless tennis there's universal boredom (5)
I believe the answer is:
ennui
'boredom' is the definition.
(I know that boredom can be written as ennui)
'in endless tennis there's universal' is the wordplay.
'in' means one lot of letters goes inside another.
'endless' says to take the centre.
'there's universal' becomes 'u' (I can't justify this - if you can you should give a lot more credence to this answer).
The centre of 'tennis' is 'enni'.
'enni' enclosing 'u' is 'ENNUI'.
Can you help me to learn more?
(Other definitions for ennui that I've seen before include "Weariness with life" , "Result of boredom" , "having nothing to do" , "tiredness" , "Effect of boredom" .)